1. Field of the Invention
This invention relates to a liquid crystal device, and more particularly to a liquid crystal device comprising an antiferro-electric liquid crystal composition showing tristable switching.
2. Description of the Prior Art
In the recent years, a ferroelectric liquid crystal display device using a ferroelectric liquid crystal such as of chiral smectic C phase has been investigated (See N. A. Clark, et al, Appl. Phys. Left., 36, 899 (1980)). The display device has a wide view angle and permits a large capacity display of more than 1000 .times.1000 lines, so that it has been expected to be promising. However, the device has such problem that good orientation and memory characteristic required for the display device cannot be readily realized in a practical cell and is likely to be influenced by a shock from the outside. The display device has to settle many tasks for realization.
On the other hand, there has been recently found a compound having a liquid crystal phase that shows tristable switching at lower temperature than that of the chiral smectic C phase above-mentioned, and a new display device related to this has been investigated (A. D. L. Chandani, et. al., Jpn. J. Apply. Phys., 27, L279 (1988)).
The newly found liquid crystal phase is not yet fully clarified and is expressed in various manner by investigators, for example, S.sub.Y * phase (Japanese Unexamined Patent Publication HEI 1-213390/1989) or SmC.sub.A * (Fukuda, the Japan Society for the Promotion of Science Information science organic materials, 142th Committee, 45th joint research materials, 34(1989)). The phase may be regarded as an antiferroelectric smectic phase having a helical structure wherein the long axes of molecules are tilted with respect to the layer plane. When the helices are set unwound, for example, by sealing the phase in a liquid crystal cell thinner than the helical pitch length, there is provided a molecular arrangement to cause dipoles to be canceled in each layer, as shown in FIG. 1(a). When electric field is applied in this state, dipoles may be changed in molecular arrangement to be aligned with each other along the direction of voltage as shown in FIGS. 1(b) or 1(c). Hence, for example, polarizing plates can be combined with the device to realize a bright or dark display. The relationship between the applied voltage and the tilt angle is as shown in FIG. 2, wherein three stable states 1 to 3 can be taken to draw a hysteresis curve. Therefore, the relationship may be used for driving liquid crystal display device.
Only a few compounds showing the above liquid crystal phase have been reported, for example, as follows. (Japanese Unexamined Patent Publication HEI 1-213390(1989); Y. Suzuki, et al., Proc, 2nd International Conference on Ferroelectric Liquid Crystals, P-106 (1989)). ##STR4##
The abovementioned optically active biphenyl derivatives may be synthesized by the method as disclosed in Japanese Unexamined Patent Publication SHO 63-307837 (1988).
Although not impossible to produce liquid crystal devices by using a single compound which shows the above antiferroelectric liquid crystal phase, such liquid crystal devices are not fully satisfactory in that the temperature range showing tristable switching is narrow and a relatively higher voltage is required for controlling the switching.